Hybrid sulfonated polyethersulfone films enabling efficient methylene blue adsorption and UV-assisted photocatalytic regeneration.

Synthetic dyes represent a major environmental concern because of their toxicity, persistence and resistance to biodegradation. Among them, methylene blue (MB) is widely used as cationic dye and may pose risks to aquatic ecosystems when improperly discharged. In this work, sulfonated polyethersulfone (sPES)-based nanocomposite films incorporating TiO2 nanoparticles, multiwalled carbon nanotubes (MWCNTs) and covalently linked MWCNTs-TiO2 nanohybrid fillers were prepared and evaluated for MB removal from water and UV-assisted regeneration. FT-IR spectroscopy and thermogravimetric analysis confirmed preservation of the sPES structure after filler incorporation, while revealing filler-dependent changes in the hydrophilic/hydrophobic interfacial environment. Batch adsorption experiments showed that MB removal is primarily governed by the sulfonated polymer matrix, with complete dye uptake under the investigated conditions and pseudo-first-order kinetics. MWCNTs alone slightly slowed adsorption, likely because of reduced film hydrophilicity and lower accessibility of internal sulfonic sites, whereas the TiO2/MWCNT formulation preserved fast adsorption, suggesting improved filler dispersion and interfacial accessibility. UV-assisted regeneration experiments demonstrated that TiO2-sPES films can bleach adsorbed MB and be reused over consecutive adsorption/regeneration cycles. The adsorption kinetics became faster after repeated cycles, attributed to swelling-induced hydration and progressive exposure of internal binding sites. Nitrate, total nitrogen and dissolved organic carbon analyses revealed that photocatalytic regeneration is strongly controlled by the MB/composite mass ratio. At low dye loading, nitrate formation reached the theoretical value expected for complete mineralization of MB-derived nitrogen within approximately 4 h at an equilibrium pH around 4, relevant to acidic industrial effluents. At higher loading, nitrogen release remained far below the theoretical value, while DOC increased more rapidly. FT-IR monitoring of highly loaded films during UV exposure showed progressive attenuation of diagnostic MB bands, indicating gradual photoconversion of adsorbed MB and involvement of lateral phenyl rings.